178 Transactions of the Royal Microseojpical Society. 
the spurious disk ought to be then one thousand times less than the 
^^0- of an inch, or tWo oo-- But when accurately measured by a 
very excellent micrometer by Browning, it measured T^hw^ nearly ; 
therefore the disk was enlarged by diffraction to twenty- five times 
its correct size. This result shows how hopeless it is to measure 
under the microscope the size of unknown brilliant objects illumi- 
nated with intense sunlight. With all the advantage of the widest 
angular aperture, which, as we have seen, diminishes the spurious 
disk proportionally, an aperture of 150° giving a much smaller 
disk than one of 70°, still the brilHant disk was at least twenty 
times too large, and this too with the finest glasses extant. 
The telescope or microscope properly handled give just the same 
evidence in kind. The latter instrument gives far the most start- 
ling results as to the limitation of vision when brilliant points are 
allowed to develope all their wonderful diffraction phenomena, but 
both instruments can be alternately employed for the purpose of 
illustrating the present limits of vision, remembering, however, 
that the best microscopes give very much smaller diffraction disks 
and rings than the very best telescopes, power for power. 
There can be little doubt that much of the clever manipulation 
of the olden times with inferior glasses owed its success to the 
patient and experienced, yet unconscious reduction of injurious 
diffraction; as well by modifying the light as its direction and 
kind. In former times, the precise position of the mirror for 
throwing the rays of reflected light at one particular angle (often 
hit only with much waste of time and labour) was attained with 
more or less success, so as to give the most brilliant definition of 
difficult objects. In 1862 I adapted a semicircular arc carrying a 
condenser, and afterwards I constructed gimbals to carry an achro- 
matic condenser at any angle of obliquity, attached to a double- 
motion stage placed exactly beneath the upper stage movements : 
by this contrivance particular angles of illumination could be 
readily attained without the excessive aberration of the usual wide- 
angled achromatic condenser. The instrument is exhibited in the 
South Kensington Museum Collection, No. 3551. Described as 
follows : — 
3551. Microscope with complex adjustments, searcher, and 
oblique condenser apparatus. Dr. Eoyston-Pigott, F.E.S. 
" This microscope is fitted with a peculiar hypocycloidal move- 
ment and traversing screws for very delicate observations. The 
condenser possesses wide rectangular movements combined with a 
unique oscillating oblique action for directing the minute image of 
a flame or of the sun either directly or obliquely upon any desired 
point in the field of view, giving fine views of many difficult 
objects, and gorgeous diffraction phenomena with circular solar 
